Hermansky-Pudlak syndrome (HPS) is a group of rare genetic diseases characterized by oculocutaneous albinism, excessive bleeding, and other symptoms that reflect defects in membrane transport processes required to generate tissue-specific lysosome-related organelles (LROs). Patients with HPS types 1, 2 or 4 additionally suffer from a lethal lung fibrosis, reflecting injury to alveolar type II epithelial cells (AT2). A2 injury correlates with defects in lamellar bodies (LBs), which are lipid-enriched LROs in which surfactant is synthesized and packaged for secretion. Despite the major importance of LBs to human health, little is known about their membrane protein composition, their derivation from endolysosomes, the membrane trafficking pathways that lead to their maturation, or how the protein complexes that are defective in HPS function within these pathways. In this basic cell biology proposal, we will define membrane transport processes required for LB biogenesis and maturation, and link them to the protein complexes that are defective in HPS - AP-3 and BLOC-1, -2 and -3. We will test the hypotheses that (1) LBs mature through distinct stages in AT2, (2) AP-3 and the BLOCs facilitate LB maturation by targeting LB-specific integral membrane cargoes from conventional endosomes to early LB stages, and (3) the trafficking pathways involved in LB biogenesis are universal to LRO-generating cell types and are independent of cargo. These hypotheses stem directly from comparisons of LB biogenesis to LRO maturation in melanocytes, in which melanosomes mature through distinct morphological stages through the AP-3- and BLOC-dependent delivery of melanogenic enzymes from early endosomes to non-pigmented precursors. Our results will further our understanding of LBs as critical LROs, aid in the development of new HPS diagnostic tools and therapies, and lay the foundation to understand the etiology of the lung fibrosis in HPS patients. Our specific aims are: 1. To test whether LBs of different densities represent distinct stages in LB maturation and if progressive cargo protein delivery shapes the lipid profile of maturing LBs. We will profile the contents of enriched, distinct LB fractions from maturing AT2, and validate our findings by immunofluorescence and immunoelectron microscopy. 2. To test whether LB cargo delivery requires AP-3 and/or AP-1. We will test for binding of the cytoplasmic domains of putative cargoes to cytoplasmic adaptors used by melanosomal proteins for sorting to melanosomes, and assess the requirement for these adaptors in cargo localization to LBs in AT2. 3. To test whether LRO maturation is universally defined by a cargo delivery pathway mediated by HPS-associated protein complexes in AT2 and melanocytes. We will test whether ectopically expressed melanosome and LB cargoes localize to LBs in AT2 and melanosomes in melanocytes, and whether localization in both cell types similarly requires BLOCs.